I. Field of the Invention
The invention relates to a process for the low-deformation diffusion welding of ceramic components, to the monoliths produced by this process and to their applications.
II. Description of the Related Art
Ceramic components are in general use in plant and mechanical engineering where wear, corrosion and high thermal loads occur. The hardness, chemical stability and high-temperature stability of ceramics is far superior to the corresponding properties of steels. Moreover, silicon carbide, as a representative of industrial ceramics, has the particular advantage of an extremely good thermal conductivity (four times better than that of steel). This predestines the material not only for use in nozzles, valves, axial face seals and sliding-contact bearings but also for use in reactors, such as for example tube bundle heat exchangers or diesel particulate filters. In many of these applications, the ceramic components have to be of very complex shape for design reasons. The design is often incompatible with available ceramic shaping processes, which makes it necessary to join individual constituents. Consequently, the literature has disclosed numerous works relating to the joining of ceramic, including many relating to the joining of SiC ceramic. Depending on the process, the literature uses the term “diffusion welding”, “reaction bonding” or “soldering”. Soldering and reaction bonding leave behind a seam in the interface between the joining partners, whereas diffusion welding can be applied in such a way that the joining partners form a seamless component. Seamless components of this type are also described as monoliths.
Fundamental principles on the theme of diffusion welding of sintered SiC components were disclosed by Thomas Moore as early as the 1980s. He demonstrates in the article “Feasibility study of the Welding of SiC” in J. Am. Ceram. Soc. 68 [6] C151-C153 (1985) that a stable, cohesive join between polished planar plates of α-SiC with the aid of diffusion welding is only possible if the temperatures and pressures applied are so high that approximately 25% plastic deformation of the components to be joined in the direction of the pressing pressure has to be accepted. The article concludes that it is not possible to produce a seamless welded join of sintered SiC without plastic deformation. Even after hot-pressing at 1950° C. and 13.8 MPa pressure (time 2 h), there are seams between the joined and considerably deformed plates. A drop in the temperature is not likely to lead to any better results with regard to a seamless join. Increasing the pressure during the diffusion welding to 138 MPa, realized with the aid of hot isostatic pressing, according to the report likewise does not produce a successful join. The inadequate cohesive joining observed between the components is attributed to the insufficient sintering activity of SiC.
U.S. Pat. No. 4,925,608 (1990) describes as a process the diffusion welding of slightly pre-sintered SiC components based on hot isostatic pressing in order to obtain a cohesive, seamless SiC bond. Here, particular emphasis is placed on the β-modification of SiC and the higher sintering activity of the components, which are still up to 85% porous. Temperatures of >1700° C. and pressures of greater than 150 MPa are preferred. Since densification of the porous components still occurs during the joining, correspondingly high degrees of plastic deformation occur.
To keep the overall levels of plastic deformation low yet nevertheless to achieve high-quality joins, the bulk of the works disclosed in the literature concentrate on the “soldering” and “reaction bonding” joining processes at significantly lower temperatures. Nowadays, the state of the art is for ceramic components to be joined with the aid of adhesives at room temperature, to be joined with the aid of metal and glass solders in the region of around approx. 1000° C. or to assemble them into components by reaction bonding at approx. 1400° C. In this context, in particular the reaction bonding of silicon-infiltrated SiC (Si—SiC) should be mentioned, a process which has been used to produce even complex components, such as plate-type heat exchangers, in the past. However, the joining seams are still a weak point of the components. Decomposition, softening or release of silicon, followed by failure, occurs here at an early stage under high thermal, corrosive or wearing loads. Even nowadays, it is considered impossible to join sintered SiC (SSiC) seamlessly and with little deformation.
Therefore, it is an object of the present invention to provide a process which allows components made from a nonoxide sintered ceramic to be joined to one another in such a way that a seamless monolith is formed and the plastic deformations during joining are kept at such a low level that the contours of the monolith already correspond to those of the desired component. There is consequently no need for a subsequent hard-working.